The present invention relates to a material for a piston ring used to obtain developed power of an internal combustion engine such as an automobile engine by maintaining airtightness of the combustion chamber, and a piston ring which is produced from this material and the surface portion of which is nitrided.
Conventionally, a cast iron has been used for piston rings. However, as is particularly noticeable with automobile engines, piston rings have also come to be required to be light in weight in conjunction with the tendency for the engines to become light. On the other hand, the tendency for the engines to produce high developed power has made the working environments of the piston rings severe. Hence, a material which can bring about high performance with respect to various properties required and which is highly durable has been in demand as the material of the piston rings. Among conventional materials, one which takes abrasion resistance and heat resistance into account is disclosed in Japanese Patent Laid-Open Publication No. 52-27011. However, since this has premised a cast piston ring, it is difficult to make the piston ring light in weight by making its wall thickness small. In addition, since this material contains high contents of C and Si in view of castability and has a micro-structure of as cast state, it is difficult for the material to bring about such fatigue strength and toughness that are presently demanded. Under such background, a so-called steel ring has been thought of which is obtained by working a steel-made flat wire into an annular shape and has recently come to be used extensively. This steel ring has advantages in that it permits the wall thickness of the piston ring to be made samll, it satisfies the demand for light weight, and the production process can be made extremely simplified as compared with that for cast iron rings. Furthermore, by working a wire product, it is possible to produce piston rings having various characteristics by selecting a material from a wide class of conventional steels in compliance with required characteristics. Because of such background, the present applicant has put such steel piston ring materials into practical use that are disclosed in, for instance, Japanese Patent Examined Publication Nos. 61-22131, 57-8302, 58-46542, and 61-21302.
At present, Si-Cr steel (JIS SWOSC-V), and JIS SKD 61, 13Cr- and 17Cr-based martensitic stainless steels are used for those steel-made piston rings for automobile engine use in which a particularly severe working condition is required. These materials are used the hardness of which is kept to be in the range of HRC 38 to 45 because of the requirements of workability of the rings. Outer peripheral portion of the ring which slides against the cylinder is subjected to hard Cr plating, composite plating including hard particles, or nitriding in the case of a high Cr material so as to improve the abrasion resistance and scuffing resistance.
In the process of development of piston rings for use in automobile engines, for the purpose of attaining light-weight, the use of cast iron rings is shifting to that of steel rings which permit the wall thickness to be made small, and, as for the steel rings, the surface treatment provided on the sliding portions for reducing their sliding abrasion with respect to the cylinder is tending to shift from Cr plating to nitriding. In conjunction with these shifts, the ring material is also shifting toward a high Cr contained steel. A 17Cr contained martensitic stainless steel (JIS SUS440 B class) is available as the material capable of obtaining the highest performance.
Recently, however, the problem of the scuffingresisting property concerning scuffing between the cylinder and the ring has occurred due to the tendency to high-output performance resulting from diesel engines and the use of turbo chargers. Conventionally, the main characteristics required of piston rings were heat resistance and abrasion resistance. As high-developed power engines have become widespread, there arises a problem of the phenomenon of scuffing between the cylinder and the piston ring which scuffing occurs during the starting of the engine and abrupt rise in the engine speed. Hence, there has been demand for a material having a higher performance than that of the piston ring of the 17Cr contained martensitic stainless steel. Two methods of surface treatment are conceivable as countermeasures for improving the aforementioned phenomenon of scuffing. A first method is to effect plating of a composite material including dispersed hard particles or to effect flame-spraying of a metal or ceramics on the outer peripheral portion of the piston ring that slides against the cylinder. This method makes it possible to improve the scuffing-resisting property and abrasion resistance, but the abrasion of the inner surface of the cylinder is intense. Moreover, this method tends to result in the exfoliation of the surface-treated layer and the deterioration of the mechanical properties (particularly the fatigue strength) of the ring material. Therefore, a further research is needed with respect to this method. A second method is to effect a nitriding treatment which is a simple process and which is effective in improving the mechanical properties of the ring material. However, if the conventional steels are used, its effect of preventing scuffing is small and is unsatisfactory.